mt76: mt7915: directly read per-rate tx power from registers

Since driver no longer handler per-rate tx power setting, we need to
read the power values directly from registers.

Tested-by: Evelyn Tsai <evelyn.tsai@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Ryder Lee <ryder.lee@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>